Watching
the wave of spring sweep northward is at the heart of Journey North. All
seasonal changes – temperature, plant growth and life cycles, animal
migrations, and so on – are driven by shifts in the amount
of available sunlight (called day length or photoperiod) and its intensity
(related to the angle at which it strikes the Earth). Here's a refresher
on key concepts, and a Season Simulator animation to demonstrate how and
why changes in photoperiod and sunlight angle occur.

Rotation
vs. Revolution

Viewed from above, the Earth makes a complete counterclockwise rotation
(spins on its axis) once in each 24-hour period. This is why the Sun appears
to rise in the east and set in the west. But this daily
rotation has nothing to do with seasons. The Earth also revolves counterclockwise
around the Sun once every 365 1/4 days. This yearly
revolution, combined with the Earth’s tilt (see below) gives us
seasons.

Earth's yearly revolution,
month-by-month.

Tilt
and Revolution

The Earth's axis is not oriented vertically, but is tilted by 23.5 degrees.
The north end of the axis is always pointed toward the North Star as the
Earth revolves around the sun. This tilt, combined with its revolution around
the Sun, causes seasonal changes. (When it’s summer in the northern
hemisphere, it’s winter in the southern, and vice versa.) If the axis
was not tilted, our year-round climate would be rather boring and many places
on Earth wouldn’t receive much light!

Season Simulator

During
our summer, the Northern Hemisphere leans toward the Sun in its
revolution, there are more daylight hours, and the Sun’s angle is
more perpendicular to us than at other times of year. The longer days
and more concentrated sunlight result in more heating. (Shadows are
shorter in the summer because the sun strikes Earth more directly.)

During
winter, the Northern Hemisphere leans away from the Sun, there
are fewer daylight hours, and the Sun hits us at an angle; this makes
it appear lower in the sky. There is less heating because the angled Sun’s
rays are “spread out” rather than direct. (Shadows are longer
because of the lower angle of the Sun.)

In equatorial regions, the length of days and the directness of sunlight
don't change as much. The further you get from the equator, the more dramatic
the seasonal changes.

During
the spring and fall, the Earth leans neither toward nor away
from the Sun; daylight and nighttime hours are more equal and temperatures
are moderate. (The shadow of an object is similar during these seasons.)

Season Simulator
Click and drag the Earth around the Sun, or
play the animation, to see what's at the base of all seasonal
change.Credit: Nebraska Astronomy Applet Project

Common
Misconceptions about the Seasons
Many students (and adults) believe that the Earth is closer to the Sun
in the summer and further away in the winter. (It’s actually somewhat
closer to the Sun in the winter, but the angled rays and short days
don’t give us much heat.) Another misconception is that the Earth
orbits the Sun in an elongated ellipse, which makes the Earth’s
distance from the Sun dramatically different at different locations.
The reality is that the Earth’s orbit is nearly circular.

Solstice
and Equinox

Solstice refers to the two times each year when the Sun's
strongest rays are furthest from the equator (north of it during our summer
solstice and south during the winter). For the northern hemisphere, summer
solstice occurs around June 21st; we have the maximum number of daylight
hours at that time. Winter solstice is around December 21st when we have
the fewest daylight hours.

Equinox refers to the two times each year when the Sun's strongest
rays are directly hitting the equator. Everywhere on Earth has 12 hours
of daylight on the spring and fall equinoxes. In the northern hemisphere,
spring equinox occurs around March 21st and autumnal equinox around
September 21st.